Method for detecting brake circuit failure

ABSTRACT

A method is provided for detecting a fault in a hydraulic brake system of a vehicle having a first hydraulic brake circuit and a second hydraulic brake. The first hydraulic brake circuit includes a pressure sensor. A pressure is measured within the first hydraulic brake circuit of the vehicle using the pressure sensor. An expected vehicle acceleration is estimated in response to the measured pressure. An actual vehicle acceleration is measured. A determination is made whether the expected vehicle acceleration is within a predetermined range of the actual acceleration. A first and second wheel differential is determined. A determination is made whether the first wheel differential and second wheel differential satisfy a predetermined condition. A fault in the hydraulic brake system is detected in response to determining whether the first and second wheel differentials satisfy a predetermined condition and in response to determining whether the actual and estimated acceleration are within the predetermined range.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to detecting a hydraulic brake circuit failure within a split vehicle braking system, and more particularly, to detecting a hydraulic brake circuit failure while requiring a pressure sensor in only one of the two hydraulic brake circuits.

2. Description of the Related Art

Vehicle braking systems commonly include a master cylinder and reservoir for providing pressurized brake fluid to hydraulic brake circuits for actuating vehicle brakes. Vehicle braking systems typically include two hydraulic brake circuits for actuating a respective set of vehicle brakes. During vehicle stability control mode, as a driver of the vehicle asserts a force on the brake pedal of a vehicle, the master cylinder (M/C) pressure is typically measured by a respective pressure sensor in each hydraulic brake circuit or other type of sensing device which determines the M/C pressure within each of the hydraulic brake circuits (e.g., pressure switch or a brake pedal travel sensor measuring the distance the brake pedal has traveled) for determining the drivers braking demands. Based on the braking demands received from the sensing devices, a motor, pump, and associated valves provide pressurized hydraulic brake fluid to the vehicle brake actuators for actuating the vehicle brakes. The pressurized hydraulic brake force applied to the vehicle brake actuators is directly correlated to the driver's braking demands (the N/C pressure as measured in both hydraulic circuits).

Some vehicle braking systems may include secondary assist brake functions. Such secondary assist brake functions provide anti-lock braking (ABS), traction control (TC), and yaw stability control (YSC) functions. These secondary assist brake functions supplement the driver actuated hydraulic brake system. For example, the ABS system pulsates the braking system if the operator of the vehicle locks the brakes so as prevent the vehicle from skidding to shorten the braking distance traveled of the vehicle. Each of the assisted brake functions provides some type of added vehicle braking or vehicle stability control utilizing either one or more brakes individually or in combination. The secondary brake assist functions require additional pressure above the driver applied N/C pressure. For example, for the YSC function, the M/C pressure is used as a reference for the pressure target of an isolated circuit. During YSC control, pressure for an isolated circuit is required to be increased above the M/C pressure (i.e., YSC+M/C pressure_(measured)). A pressure controller and pressure estimate is used to deliver the required pressure to the wheels of the isolated circuit for applying the secondary assist brake function. These functions typically are automatically activated usually without awareness of the driver.

When a hydraulic brake circuit failure is detected, secondary assist braking functions, such as anti-lock braking, traction control, yaw stability control, may be deactivated for allowing the operator of the vehicle to slow down or stop the vehicle on its own without any assistance from secondary assist braking functions. The reason is to prevent any automated braking actions, which are activated by the secondary assist braking functions, from interfering with the driver's intended braking demands. For example, if the hydraulic brake failure is occurring, and the driver wants to stop immediately, the assisted braking cannot sense this issue or condition the driver is faced with and may attempt a vehicle stability control operation that is not in cooperation with the drivers intended braking operation.

To detect a hydraulic brake circuit failure a pressure sensor is provided for each hydraulic brake circuit in the vehicle. Typically there are two hydraulic brake circuits for providing hydraulic brake fluid to a respective wheel or sets of wheels. In a two hydraulic brake circuit system, each hydraulic brake circuit provides hydraulic brake fluid to a respective pair of vehicle brakes and two pressure sensors are often utilized. Alternatively, in the two hydraulic brake circuit system, a pressure sensor could be utilized on a first hydraulic brake circuit and a pressure switch could be utilized on the second hydraulic brake circuit. Each pressure sensor or switch provides a signal indicative of the fluid pressure within a respective circuit.

To properly utilize the assisted brake functions, it must be known whether both hydraulic brake circuits are functioning properly, otherwise the assisted brake functions could ignore the drivers intended braking demands. If only one pressure sensor was utilized within a braking system utilizing two hydraulic brake circuits, then such a failure could go undetected. For example, if a failure occurred in the sensed circuit, the pressure sensor measuring pressure on the sensed line would provide a M/C pressure measurement of approximately zero. The unsensed circuit, in this example, would have a M/C pressure greater than zero. However, without a second sensor (or other type of indicator) in the unsensed circuit for comparing the pressure within the two hydraulic brake circuits, a potential fault condition may go undetected and braking system would not recognize the drivers braking demands as the M/C pressure is zero in the sensed circuit. As a result, minimum or no braking force would be applied to the vehicle brakes in YSC control modes.

In the event of a failure of an unsensed line, the pressure sensor would measure the pressure applied within the sensed circuit (non-failed circuit) and would provide a signal to the motor, pump and associated valves to supply pressurized hydraulic brake fluid to the vehicle brake actuators. However, zero or minimal pressure will be generated in the unsensed (failed) circuit, and as a result, only 50 percent of the vehicle braking would be applied.

The hydraulic brake failure would remain undetected in both situations without some secondary method of verifying the hydraulic brake failure, secondary assist brake functions would remain active. Such braking function could potentially conflict with the driver's immediate braking demands. Thus, in YSC systems, it is critical to utilize a pressure sensor in each hydraulic brake circuit, or other device (e.g., pressure differential switch) to detect the failure, however, the addition of a pressure sensor for each additional circuit in a braking system becomes costly.

SUMMARY OF THE INVENTION

The present invention has the advantage detecting a fault in a hydraulic braking system utilizing at least two hydraulic brake circuits and only one pressure sensor for a respective hydraulic brake circuit and for deactivating at least one assisted brake function in response to detecting said fault.

In one aspect of the present invention, a method is provided for detecting a fault in a hydraulic brake system of a vehicle having a first hydraulic brake circuit for actuating a first set of vehicle brake actuators for a first set of wheels and a second hydraulic brake circuit for actuating a second set of vehicle brake actuators for a second set of wheels. The first hydraulic brake circuit includes a pressure sensor. A pressure is measured within the first hydraulic brake circuit using the pressure sensor. An expected vehicle acceleration is estimated in response to the measured pressure. An actual vehicle acceleration is determined. A fault is detected in response to a comparison of said actual acceleration and said expected acceleration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a hydraulic braking system according to the preferred embodiment of the present invention.

FIG. 2 is a block diagram of a system for detecting a failed hydraulic brake circuit according a preferred embodiment of the present invention.

FIG. 3 is method for detecting a fault in a hydraulic brake system according to a first preferred embodiment of the present invention.

FIG. 4 is method for detecting a fault in a hydraulic brake system according to a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the Drawings and particularly to FIG. 1, there is shown a schematic diagram of a hydraulic braking system for providing hydraulic brake fluid to a plurality of vehicle brake actuators. The hydraulic braking system is shown generally at 10. The hydraulic braking system 10 includes vehicle wheels 11 a, b, c, and d. Vehicle brake actuators 21 a, b, c, and d each include a respective brake actuation member (such as a slave cylinder) and friction member actuable by the actuation member for engaging a rotatable braking surface of the vehicle wheels 11 a, b, c, and d, respectively. In the preferred embodiment, the vehicle braking system utilizes a diagonally split braking system. A first circuit of pressurized hydraulic brake fluid 12 is provided for actuating vehicle brake actuators 21 a and 21 d. The second circuit of pressurized hydraulic brake fluid 14 is provided for actuating vehicle brake actuators 21 b and 21 c. In other preferred embodiments, a respective hydraulic brake circuit may actuate an axial set of vehicle brake actuators.

The source of pressurized brake fluid for the first circuit 12 and second circuit 14 is a manually operated master cylinder 16. The master cylinder is operated by a brake pedal to supply pressurized brake fluid to the first circuit 12 and the second circuit 14. Typically the master cylinder 16 includes a tandem master cylinder, having two service pistons, but the master cylinder 16 may be of any suitable design.

A vehicle brake module 18 may include a vehicle stability control module in addition to a hydraulic braking boost system. The vehicle stability control (VSC) module may comprise ABS functionality, TC functionality, or YSC functionality. The vehicle brake module 18 receives pressurized hydraulic brake fluid from the first circuit 12 and the second circuit 14. The vehicle brake module 18 outputs pressurized hydraulic brake fluid to the respective vehicle brake actuators via the first circuit 12 and the second circuit 14. Various hydraulically controlled valves 30 (shown in FIG. 2) within the vehicle brake module 18 control the hydraulic pressure to the four vehicle brake actuators 21, a, b, c, and d either independently or in combination for performing various deceleration or stability control operations. A pressure sensor 20 is mounted to the first circuit 12 to sense pressure within the first circuit 12, or alternatively, the pressure sensor 20 may be mounted to the second circuit 14 for sensing pressure within the second circuit 14.

FIG. 2 illustrates block diagram for detecting a failed hydraulic brake circuit. The vehicle brake module 18 includes a plurality of controls for controlling the actuation of the vehicle brakes 21 a, b, c, and d. In the preferred embodiment, the vehicle brake module 18 includes a controller 22 for receiving input signals from a plurality of sensing devices. The plurality of sensing devices includes a pressure sensor 20, an acceleration sensor 34, and a wheel speed sensor 32.

The pressure sensor 20 senses the hydraulic brake pressure in the first circuit 12 and inputs a pressure signal to the controller 22. In the preferred embodiment, a plurality of sensors is used to determine both wheel speed and acceleration. A wheel speed sensor 32 senses the velocity of a wheel. A respective wheel speed sensor is disposed at each wheel location for sensing the velocity of a respective wheel. Preferably, the wheel speed sensors are used to determine the vehicle acceleration by measuring vehicle speed over various increments of time. Utilizing the wheel speed sensors to determine vehicle acceleration minimizes the complexity and cost of adding an additional sensor for measuring acceleration. Alternatively, an additional sensor such as an acceleration sensor 34 may be used to sense the acceleration of the vehicle. Acceleration may be a negative acceleration (i.e. deceleration) or a positive acceleration. In other preferred embodiments, the sensed acceleration may be supplied by other devices sensing the acceleration such as a powertrain control module which receives a sensed acceleration input from a driveshaft of the vehicle. The parameters sensed from the plurality of sensors are input to the controller 22 for determining a hydraulic brake failure in the sensed and unsensed hydraulic brake line.

The vehicle brake module 18 includes a plurality of vehicle stability modules for applying one or more secondary brake assist functions. The plurality of vehicle stability modules includes an ABS module 24 for operating in an anti-lock braking state, a TC module 26 for operating in a traction control state, and a YSC module 28 for operating in a yaw stability state. The plurality of vehicle stability modules provide signals to electrically controlled hydraulic brake valves 30 for modulating the braking pressure of each hydraulic circuit so to apply various braking and vehicle stability control strategies to the vehicle. The master cylinder 16 provides pressurized hydraulic brake fluid to the hydraulic brake valves 30 which are open and closeable depending on the braking strategy applied from the various controllers. In alternative embodiments, the plurality of vehicle stability controllers that control the assisted braking functions may be located exterior of the assisted braking module. If exterior, a communication line is coupled between a respective vehicle stability module and the vehicle brake module 18. In addition, a respective set of hydraulic brake valves for providing secondary brake assist functions for a respective vehicle stability function (e.g., ABS) may include a separate set of hydraulic valves disposed exterior of the vehicle brake module 18. This separate set of hydraulic valves would cooperatively work with hydraulic brake valves 30 to apply the respective braking function.

In the preferred embodiment, the pressure sensor 20 is connected to the first circuit 12 for measuring pressure within the first circuit 12. A hydraulic brake failure in the first circuit 12 would result in a zero pressure measurement (or minimal pressure) of the first circuit 12. Without a method for determining the hydraulic brake circuit failure, the system would view the zero pressure as a no brake applied condition and the driver demand pressure would not added to the second circuit applying YSC control functionality. This would result in ignoring the driver demanded braking. If a hydraulic brake failure occurs in the second hydraulic brake circuit 14, then the hydraulic brake pressure will be zero in the second circuit 14. However, since the pressure sensor 20 is measuring pressure in the circuit which it is attached to (i.e., the first circuit 12), there will be no indication of a failure. Driver demand would be applied to the both circuits, however, the second circuit 14 may generate minimal or no braking pressure. To determine that a hydraulic brake failure in either circuit has occurred without adding a pressure sensor or pressure switch to the second circuit 14 and comparing the measured pressure of both circuits, other vehicle operating parameters are utilized.

The acceleration sensor 34 provides actual vehicle acceleration measurements to the controller 22. Other devices (not shown) such as the powertrain control module, wheel speed sensor, or accelerometers may also be used to provide data to the controller 22 to determine acceleration. The acceleration data provides information as to whether the vehicle is decelerating (negative acceleration). The controller 22 also receives input data from the pressure sensor 20 that provides the measured hydraulic pressure from the first circuit 12. A correlation ratio is determined by the controller 22 based on the input data. The correlation ratio is represented by the following formula: C _(f) =P _(m) /A _(x) where C_(f) is the correlation factor, P_(m) is the measured pressure of the first circuit, and A_(x) is the measured vehicle acceleration (negative or positive).

This ratio indicates that for a respective negative acceleration (i.e., deceleration) the first hydraulic brake circuit 12 should indicate a respective increased pressure from the pressure sensor 20. This assumes that for a given amount of force exerted on the brake pedal, a resulting hydraulic pressure will be produced in a respective hydraulic brake circuit and a respective negative vehicle acceleration will be produced in response to the applied braking action unless there is a failure of pressure in the other circuit. Thus, for a given pressure, the vehicle should be decelerating within a respective predetermined range.

A hydraulic brake failure is determined based on whether the correlation ratio is within or outside of a predetermined range. For example, if the correlation ratio increases greater than 50%, then a determination is made that the second circuit 14 without the pressure sensor 20 could be failed. Alternatively, if the correlation factor decreases by greater than 50% a determination can be made that the first circuit 12 with the pressure sensor 20 could be failed.

In one preferred embodiment, a method is provided for estimating an expected vehicle acceleration based on the measured brake pressure. The estimated vehicle acceleration is then compared to the actual vehicle acceleration for detecting a fault in one of the hydraulic brake circuits.

A determination for detecting the fault in the hydraulic brake system may be based strictly on the determining the correlation ratio (or comparing estimated and actual acceleration); however, other factors aside from applying the vehicle brakes may be contribute to the vehicle having a negative acceleration without an expected increase in the pressure of one or both of the hydraulic brake circuits. Such factors include traveling up a steep incline without applying increased pressure on the accelerator to maintain velocity or shifting to a lower gear or neutral which would cause a negative acceleration without applying any braking force.

To increase the confidence level that a potential fault is occurring, a further determination is made whether a first and second wheel differential satisfy a predetermined condition. The first wheel differential is determined between a first wheel of a first set of wheels and a first wheel of a second set of wheels. A second wheel differential is determined between a second wheel of a first set of wheels and a second wheel of a second set of wheels. Wheel differential may be determined in response to a delta wheel slip between respective wheels. For a diagonal split system, the delta slips for a front set of wheels and a rear set of wheels are as follows: Delta slip_(front)=(V _(lf) −V _(rf))/V _(lf) and Delta slip_(rear)=(V _(lr) −V _(rr))/V _(lr) where V_(lf) is the velocity of the left-front wheel, V_(lr) is the velocity of the left-rear wheel, V_(rf) is the velocity of the right-front wheel, and V_(rr) is the velocity of the right-rear wheel.

If a hydraulic failure is occurring in one of the two hydraulic circuits, then the failed circuit will not receive the braking demands as desired by the driver and only the non-failed circuit will have braking pressure. Since this will cause slip on the wheels of the non-failed circuit due to the braking pressure applied, two respective wheels (each wheel from a different braking circuit) will be traveling at different velocities. Based on the delta slip calculations above, two respective wheels from the different braking circuits will result in a negative delta slip and the two other respective wheels from the different braking circuits will result in a positive delta slip. The following formula is one example of satisfying a predetermined condition in determining whether a hydraulic brake failure is occurring based on the front and rear delta slips: Delta slip_(front)*Delta slip_(rear)<0 Based on the product of the above formula, the product of an axle having a negative delta slip and an axle having a positive delta slip will be negative. Therefore, a hydraulic brake failure is assumed when the product is negative. However, other factors may contribute to the vehicle having a negative wheel slip while a hydraulic brake failure is occurring. An example of when a hydraulic brake circuit failure is present but the resulting delta slip product is positive is when a vehicle is turning.

Both methods may be used independently to determine a hydraulic brake pressure fault, however, by filtering the results of each method so as to satisfy both conditions (circuit pressure versus acceleration and wheel slip differential) increases the confidence level for detecting a failed brake circuit. If the failed brake circuit is detected, then one or more of the vehicle stability functions of the vehicle brake module 18 (such as the ABS 24, TC 26, and YSC 28) are deactivated so that such automated functions will not interfere with the driver's braking demands.

In another preferred embodiment, a wheel velocity differential between a respective pair of wheels may be used to determine wheel, as opposed to the delta wheel slips, for determining the predetermined condition for detecting the potential fault. The following formula may be used for determining a respective wheel velocity differential for a respective pair of wheels: Wheel Velocity Differential_(front)=(V _(lf) −V _(rf)) and Wheel Velocity Differential_(rear)=(V _(lr) −V _(rr)) Based on the wheel velocity differential calculations above, two respective wheels from the different braking circuits will result in a negative velocity differential and the two other respective wheels from the different braking circuits will result in a positive wheel velocity differential. The following formula is another example of satisfying a predetermined condition in determining whether a hydraulic brake failure is occurring based on the front and rear delta slips: Wheel Velocity Differential_(front)*Wheel Velocity Differential_(rear)<0 Based on the product of the above formula, the product of an axle having a negative wheel velocity differential and an axle having a positive wheel velocity differential will be negative. Therefore, a hydraulic brake failure is assumed when the product is negative.

FIG. 3 illustrates method for detecting a fault in a hydraulic brake system of a vehicle that includes two hydraulic brake circuits each actuating a respective set of brake actuators where the system includes only one pressure sensor for sensing pressure within one of the two hydraulic brake circuits. In step 30, a routine is initiated for determining the fault in the hydraulic brake system. In step 31, the pressure of respective hydraulic circuit is determined from a pressure sensor sensing the respective hydraulic circuit. In step 32, the actual vehicle acceleration is determined. The actual vehicle acceleration may be determined from an accelerometer, wheel speed sensor, or input from the powertrain control module.

In step 33, a correlation ratio is determined and is defined by the ratio of the measured pressure of the respective hydraulic circuit versus the actual vehicle acceleration. In step 34, a determination is made whether the correlation ratio is within a predetermined range. If a determination is made that the correlation ratio is within the predetermined range, then a determination is made that a hydraulic brake circuit failure is not occurring and a return is made to step 31 to continuously sense for a hydraulic brake circuit failure.

If a determination is made that the correlation ratio is not within the predetermined range, then the delta slip of the front wheels are determined in step 35. In step 36, the delta slip of the rear wheels is determined. In step 37, a determination is made whether the front delta slip and the rear delta satisfy a predetermined condition. An example of satisfying the predetermined condition is to multiply the front wheel delta slip and the rear wheel delta slip and determine if the sign of the product is negative. If the product is positive, then a determination is made that the hydraulic brake circuit failure is not occurring and a return is made to step 31 to continuously sense for the hydraulic brake circuit failure. If a determination is made in step 37 that the product is negative, then a determination is made that the hydraulic brake circuit is faulted. In step 38, at least one of the vehicle stability functions is disabled so that the automated vehicle stability functions do not interfere with the driver's intended braking operation. In step 39, the routine is exited differential.

FIG. 4 illustrates another preferred embodiment for determining a hydraulic brake circuit failure utilizing only one pressure sensor in a braking system utilizing at least two hydraulic brake circuits. In step 40, two routines are initiated for determining the fault in a hydraulic brake system. In step 41, the first routine includes determining the pressure of respective hydraulic brake circuit based on the output of the pressure sensor. In step 42, an expected acceleration is estimated. In step 43, the actual vehicle acceleration is compared to the vehicle estimated acceleration. In step 44, a determination is made whether the actual vehicle acceleration and the vehicle estimated acceleration are within a predetermined range.

In a second routine running simultaneously with the first routine, the delta slip of the front wheels is determined in step 45. In step 46, the delta slip of the rear wheels is determined. The sign of the product of delta slips is determined in step 47 by multiplying the front wheel delta slip and the rear wheel delta slip for determining whether a predetermined condition is satisfied. If the product is positive, then a determination is made for the second routine that the hydraulic brake circuit is not faulted. If the product is negative, then a determination is made for the second routine that the hydraulic brake circuit is faulted. In step 48, the actual and estimated acceleration comparison and the products of the delta slips are filtered to cooperatively determine if a hydraulic brake circuit failure is occurring. The filter process includes determining if each routine have satisfied their conditional requirements for determining hydraulic brake failure. If the determination was made in step 44 that the actual vehicle acceleration and the estimated vehicle acceleration are not within the predetermined range and if the determination was made in step 47 that the product of the delta slips was negative, then the filter generates a determination that a hydraulic brake circuit fault is occurring and at least one of the vehicle stability functions are deactivated in step 49. The routine is then exited in step 50. Alternatively, a determination of whether both delta slips have opposite signs may be used to determine if the predetermined conditions are satisfied as opposed to multiplying both delta products and producing a product.

If the filter determined, in step 48, that either the product of the delta slips was positive or that the actual vehicle acceleration and estimated vehicle acceleration were within the predetermined range, then a return is made to steps 41 and 45 to continuously sense for a hydraulic brake circuit failure.

In accordance with the provisions of the patent statutes, the principle and mode of operation of the present invention have been explained and illustrated in its preferred embodiment. However, it must be appreciated that the present invention can be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. 

1. A method for detecting a fault in a hydraulic brake system of a vehicle having a first hydraulic brake circuit for actuating a first set of vehicle brake actuators for a first set of wheels and a second hydraulic brake circuit for actuating a second set of vehicle brake actuators for a second set of wheels, said first hydraulic brake circuit including a pressure sensor, said method comprising the steps of: measuring a pressure within said first hydraulic brake circuit of said vehicle using said pressure sensor; estimating an expected vehicle acceleration in response to said measured pressure; determining an actual acceleration of said vehicle; and detecting said fault in response to a comparison of said actual acceleration and said expected acceleration.
 2. The method of claim 1 wherein said vehicle includes a vehicle brake module, said method further comprising the step of deactivating brake modulation by said vehicle brake module in response to detecting said fault.
 3. The method of claim 1 wherein said comparison of said actual acceleration and said expected acceleration includes determining whether said actual acceleration and said estimated acceleration are within a predetermined range.
 4. The method of claim 1 further comprising the step of determining a first wheel differential between a first wheel of said first set of wheels and a first wheel of said second set of wheels and a second wheel differential between a second wheel of said first set of wheels and a second wheel of said second set of wheels.
 5. The method of claim 4 wherein said step of detecting said fault is further responsive to said first wheel differential and said second wheel differential satisfying a predetermined condition.
 6. The method of claim 5 wherein said first wheel differential and said second wheel differential are opposite signs.
 7. The method of claim 5 wherein said satisfying predetermined conditions includes multiplying said first wheel differential and second wheel differential and producing a negative result.
 8. The method of claim 4 wherein said first wheel differential comprises a comparison of a velocity of said first wheel of said first set of wheels with a velocity of said first wheel of said second set of wheels.
 9. The method of claim 8 wherein said second wheel differential comprises a comparison of a velocity of said second wheel of said first set of wheels with a velocity of said second wheel of said second set of wheels.
 10. The method of claim 4 wherein said first wheel differential comprises a first delta slip, said first delta slip being determined in response to a difference between a velocity of said first wheel of said first set of wheels and a velocity of said first wheel of said second set of wheels divided by said velocity of said first set of wheels.
 11. The method of claim 10 wherein said second wheel differential comprises a second delta slip, said second delta slip being determined in response to a difference between a velocity of said second wheel of said first set of wheels and a velocity of said second wheel of said second set of wheels divided by said velocity of said second set of wheels.
 12. A method for detecting a fault in a hydraulic brake system of a vehicle having a first hydraulic brake circuit for actuating a first set of vehicle brake actuators for a first set of wheels and a second hydraulic brake circuit for actuating a second set of vehicle brake actuators for a second set of wheels, said first hydraulic brake circuit including a pressure sensor, said method comprising the steps of: measuring a wheel velocity of each wheel of said vehicle; determining a first wheel differential between a first wheel of said first set of wheels and a first wheel of said second set of wheels; determining a second wheel differential between a second wheel of said first set of wheels and a second wheel of said second set of wheels; and detecting said fault in response to said first wheel differential and said second wheel differential satisfying a predetermined condition.
 13. The method of claim 12 wherein said first wheel differential and said second wheel differential are opposite signs.
 14. The method of claim 12 wherein said satisfying a predetermined condition includes multiplying said first wheel differential and second wheel differential and producing a negative result.
 15. The method of claim 12 wherein said vehicle includes a vehicle brake module, said method further comprising the step of deactivating brake modulation by said vehicle brake module in response to detecting said fault.
 16. The method of claim 12 wherein said first wheel differential comprises a comparison of a velocity of said first wheel of said first set of wheels with a velocity of said first wheel of said second set of wheels, and wherein said second wheel differential comprises a comparison of a velocity of said second wheel of said first set of wheels with a velocity of said second wheel of said second set of wheels.
 17. The method of claim 12 wherein said first wheel differential comprises a first delta slip, said first delta slip being determined in response to a difference between a velocity of said first wheel of said first set of wheels and a velocity of said first wheel of said second set of wheels divided by said velocity of said first set of wheels, and wherein said second wheel differential comprises a second delta slip, said second delta slip being determined in response to a difference between a velocity of said second wheel of said first set of wheels and a velocity of said second wheel of said second set of wheels divided by said velocity of said second set of wheels.
 18. The method of claim 12 further comprising the steps of: measuring a pressure; estimating an expected vehicle acceleration; and comparing an actual vehicle acceleration and said expected vehicle acceleration; wherein said step of detecting said fault is further responsive to said comparison of said actual acceleration and said expected acceleration.
 19. A method for detecting a fault in a hydraulic brake system of a vehicle having a first hydraulic brake circuit for actuating a first set of vehicle brake actuators for a first set of wheels and a second hydraulic brake circuit for actuating a second set of vehicle brake actuators for a second set of wheels, said first hydraulic brake circuit including a pressure sensor, said method comprising the steps of: measuring a pressure within said first hydraulic brake circuit of said vehicle using said pressure sensor; estimating an expected vehicle acceleration in response to said measured pressure; determining an actual vehicle acceleration of said vehicle; determining if said expected vehicle acceleration is within a predetermined range of said actual acceleration; providing whether said vehicle acceleration and said actual vehicle acceleration are within said predetermined range, then determining a first wheel differential between a first wheel of said first set of wheels and a first wheel of said second set of wheels and determining a second wheel differential between a second wheel of said first set of wheels and a second wheel of said second set of wheels; and detecting said fault in response to said first wheel differential and said second wheel differential satisfying a predetermined condition.
 20. The method of claim 19 wherein said first wheel differential and said second wheel differential are opposite signs.
 21. The method of claim 20 wherein said satisfying predetermined conditions includes multiplying said first wheel differential and second wheel differential and producing a negative result.
 22. The method of claim 19 wherein said vehicle includes a vehicle brake module, said method further comprising the step of deactivating brake modulation by said vehicle brake module in response to detecting said fault.
 23. A method for detecting a fault in a hydraulic brake system of a vehicle having a first hydraulic brake circuit for actuating a first set of vehicle brake actuators for a first set of wheels and a second hydraulic brake circuit for actuating a second set of vehicle brake actuators for a second set of wheels, said first hydraulic brake circuit including a pressure sensor, said method comprising the steps of: measuring a wheel velocity of each wheel; determining a first wheel differential between a first wheel of said first set of wheels and a first wheel of said second set of wheels; determining a second wheel differential between a second wheel of said first set of wheels and a second wheel of said second set of wheels; determining if said first wheel differential and said second wheel differential are opposite signs; if said first wheel differential and said second wheel differential are opposite signs, then measuring a pressure within said first hydraulic brake circuit of said vehicle using said pressure sensor; estimating an expected vehicle acceleration in response to said measured pressure; determining an actual vehicle acceleration of said vehicle; determining if said expected vehicle acceleration is within a predetermined range of said actual acceleration; and detecting said fault in response to said determination of said estimated vehicle acceleration and said actual vehicle acceleration within said predetermined range.
 24. The method of claim 23 wherein said vehicle includes a vehicle brake module, said method further comprising the step of deactivating brake modulation by said vehicle brake module in response to said detection.
 25. The method of claim 23 wherein said first wheel differential comprises a comparison of a velocity of said first wheel of said first set of wheels with velocity of said first wheel of said second set of wheels, and wherein said second wheel differential comprises a comparison of a velocity of said second wheel of said first set of wheels with a velocity of said second wheel of said second set of wheels.
 26. The method of claim 23 wherein said first wheel differential comprises a first delta slip, said first delta slip being determined in response to a difference between a velocity of said first wheel of said first set of wheels and a velocity of said first wheel of said second set of wheels divided by said velocity of said first set of wheels, and wherein said second wheel differential comprises a second delta slip, said second delta slip being determined in response to a difference between a velocity of said second wheel of said first set of wheels and a velocity of said second wheel of said second set of wheels divided by said velocity of said second set of wheels.
 27. A method for detecting a fault in a hydraulic brake system of a vehicle having a first hydraulic brake circuit for actuating a first set of vehicle brake actuators for a first set of wheels and a second hydraulic brake circuit for actuating a second set of vehicle brake actuators for a second set of wheels, said first hydraulic brake circuit including a pressure sensor, said method comprising the steps of: measuring a pressure within said first hydraulic brake circuit of said vehicle using said pressure sensor; determining an actual vehicle acceleration of said vehicle; determining a correlation ratio between said pressure and said actual vehicle acceleration; determining whether said correlation ratio is within a first predetermined range, measuring a wheel velocity of each wheel; determining a first wheel differential between a first wheel of said first set of wheels and a first wheel of said second set of wheels; determining a second wheel differential between a second wheel of said first set of wheels and a second wheel of said second set of wheels; determining whether said first wheel differential and said second wheel differential are opposite signs; detecting said fault in response to said step of determining said first wheel differential and said second wheel differential are opposite signs and in response to said step of determining whether said correlation ratio is within said predetermined range.
 28. The method of claim 27 wherein said vehicle includes a vehicle brake module, said method further comprising the step of deactivating brake modulation by said vehicle brake module in response to detecting said fault.
 29. The method of claim 27 wherein said step of determining whether said correlation ratio is within a predetermined range correlation ratio includes a comparison between said expected vehicle acceleration and said actual vehicle acceleration.
 30. The method of claim 27 wherein said step of deactivating said brake modulation includes deactivating an anti-lock brake function.
 31. The method of claim 27 wherein said step of deactivating said brake modulation includes deactivating a traction control function.
 32. The method of claim 27 wherein said step of deactivating said brake modulation includes deactivating a yaw stability function.
 33. A vehicle control system comprising: a braking control system including an vehicle brake module that is operable in at least one operating state; a first circuit of pressurized brake fluid for providing pressurized hydraulic brake fluid to a first set of brake actuators; a second circuit of pressurized brake fluid for providing pressurized hydraulic brake fluid to a second set of brake actuators; a plurality of sensors for monitoring wheel velocity and acceleration; and a pressure sensor for monitoring a pressure within said first circuit of pressurized brake fluid; wherein said vehicle brake module determines an estimated acceleration based on said pressure within said first hydraulic brake circuit and compares to said estimated acceleration to an actual acceleration, wherein said vehicle brake module further determines a first wheel differential and a second wheel differential from said wheel velocity, and wherein said vehicle brake module deactivates said at least one operating state in response to said first wheel differential and said second wheel differential satisfying a predetermined condition and said comparison of said actual acceleration and said estimated acceleration.
 34. The vehicle control system of claim 33 wherein said first wheel differential and said second wheel differential are opposite signs.
 35. The method of claim 33 wherein said satisfying predetermined conditions includes multiplying said first wheel differential and second wheel differential and producing a negative result.
 36. The vehicle control system of claim 33 wherein said at least one operating state includes a anti-lock braking state.
 37. The vehicle control system of claim 33 wherein said at least one operating state includes a traction control state.
 38. The vehicle control system of claim 33 wherein said at least one operating state includes a yaw stability state.
 39. A hydraulic brake circuit fault detection system comprising: a braking control system including an vehicle brake module that is operable in at least one operating state; a pressure sensor for monitoring a hydraulic brake circuit; and a controller for receiving a plurality of vehicle operating inputs, said controller determining a wheel slip of a first set of wheels and a second set of wheels and an expected acceleration in response to said received inputs; wherein said controller compares said expected acceleration with an actual vehicle acceleration, wherein said controller determines a first wheel differential between a first wheel of said first set of wheels and a first wheel of said second set of wheels; wherein said controller determines a second wheel differential between a second wheel of said first set of wheels and a second wheel of said second set of wheels; wherein said controller determines whether said first wheel differential and said second wheel differential satisfy a predetermined condition, and wherein said controller detects a fault in response to said determination of satisfying said predetermined condition and in response to said comparison of said expected acceleration and said actual acceleration.
 40. The detection system of claim 39 wherein said satisfying predetermined conditions includes multiplying said first wheel differential and second wheel differential and producing a negative result.
 41. The detection system of claim 39 wherein said controller deactivates said vehicle brake module in response to detecting said fault.
 42. The detection system of claim 39 wherein said vehicle brake module includes at least one operating state for controlling said vehicle braking.
 43. The detection system of claim 42 wherein said at least one operating state includes an anti-lock braking state.
 44. The detection system of claim 42 wherein said at least one operating state includes a traction control state.
 45. The detection system of claim 42 wherein said at least one operating state includes a yaw stability control state. 